ZM-5A 8/500. Review from the reader Radozhiva

Review of the ZM-5A 8/500 mirror-lens telephoto lens and sample photos from it specially for Radozhiva, prepared Rodion Eshmakov.

ZM-5A 8/500

ZM-5A 8/500

ZM-5A refers to super-telephoto mirror lenses. Lenses like this attract today with their huge focal length and compactness. The review presents the Soviet ZM-5A Mirror-Meniscus lens, widely distributed in the secondary market (a review of its lightweight modification ZM-5CA was already, without multilayer enlightenment, lenses with multilayer enlightenment have the prefix MC in the name). Due to the specifics of the lens, a lot of attention is paid to the history of its development in the review.

Lens specifications [taken from here]
Optical design: Maksutov-Cassegrain with field corrector (mirror lens)

ZM-5A 8/500

ZM-5A 8/500

Field of view angle (Kf = 1): 5 degrees
Resolution (center / edge, lin / mm): 40/20
Light transmittance: 0,7
Light scattering coefficient: 0,03
Filter thread: M77 * 0,75 mm
Weight: 1,225 kg
Features: interchangeable A-adapter - allows you to install shanks with M42 / M39 thread or K, N bayonets; constant relative aperture due to lack of iris - F / 8.

History of the development of mirror lenses

The whole family of Soviet mirror-lens lenses (ZLO) was born to the Soviet optician D. D. Maksutov, who in 1941 invented the meniscus scheme. The impetus for this was the attempt to develop a school telescope, which should have good image quality, ease of construction and operation, durability and low cost. The schemes developed by that time (mainly refractor achromat, Newton's reflector) did not meet the specified criteria: refractors were inconvenient to use due to their dimensions and were quite expensive, and their image suffered from chromatism. Newton’s specular telescopes were difficult to operate, since they had an open (and therefore prone to contamination, etc.) pipe and, moreover, required adjustment of the secondary mirror before each observation — it was not rigidly fixed. Hence, the idea was to develop a telescope - compact and not suffering from chromatism, like a reflector, and durable, easy to maintain - like a refractor.

Having investigated the possible options, Maksutov found that the addition of a full-aperture achromatic meniscus makes it possible to correct, to one degree or another, practically all distortions of the spherical main mirror. Moreover, it turned out to be possible to apply a mirror coating on the meniscus - a secondary mirror ("patch" on the front lens), which made it rigid and eliminated the need for adjustment before use.

Applying the invention to the existing Cassegrain mirror scheme, Maksutov developed a number of ZLOs, among which were the TMSh school telescope, MTO lenses (meniscus telephoto lenses), and much more, up to ZL micro lenses. The main advantages of the new lenses are compactness, ease of operation and maintenance, lack of chromatism, low cost (available spherical mirrors are used).

This seems surprising, but the exact same idea with a correcting meniscus was also applied in the Tahir scheme (D.S. Volosov) - one thick meniscus lens corrects almost all achromat-doublet aberrations with an air gap (see Tair-3 diagram, FS-2), which makes Taira probably the simplest anastigmatic lens.

D. D. Maksutov and TMSh - “Maksutov’s school telescope”

D. D. Maksutov and TMSh - “Maksutov’s school telescope”

Design features of ZM-5A

MTO telephoto lenses and newer ZM lenses have an additional element in their circuit - a field corrector. In MTO lenses, this is a negative bonding of two lenses, which “straightens” the lens field and reduces vignetting. In 2M lenses, the component already consists of XNUMX separate lenses and performs the same functions. By removing the corrector, it is possible to improve the image quality in the center of the frame and increase the aperture ratio, however, the vignette will grow and the resolution along the edge will deteriorate. Also a side effect of removing the corrector is the loss of the ability to focus on infinity, which can be fixed by removing the stoppers of the helicoid.

As a rule, Soviet EVLs often have defects caused by improper assembly (there is a known case when Maksutov himself rejected one of the lots of lenses entirely - and what happened when there was no one to reject?) - almost all lenses have a tightened mirror and meniscus. Therefore, it is necessary to disassemble the lens and loosen the locking nuts and screws. Such an operation carried out with an MTO-1000 lens (it is very often used as a telescope), which was not able to show anything other than the Moon, compared it with a modern 90/1250 telescope with a similar scheme - the MTO-1000 began to show stars with dots, details of the atmosphere of Jupiter, etc.

ZM-5A

ZM-5A

Let us return directly to the consideration of the ZM-5A construct. A retractable hood immediately catches your eye, which hides the focus ring when folded. For such a FR, this lens hood is short - for a 500 mm lens, it should be much longer. Even Tair-3A has more. In general, the hood is more likely to interfere, because she copes with her duties unsatisfactorily, but at the same time limits access to the focus ring. The ZM-5CA has no hood.
Also, the ZM-5A has a tripod mount with 1/4 ”and 3/8” threads (the ZM-5CA does not have a tripod mount) - which is very convenient, for example, when using the lens as a telescope for observation.

ZM-5A has a wide focusing ring with a large rotation angle. A feature of the focusing mechanism is the overrun for infinity. Its purpose is related to the thermal stabilization of the lens: when the temperature changes due to thermal expansion, infinity “slides out”. Thermal stabilization is associated with one of the most important rules for using Maksutov's EVA: the lens needs up to 40 minutes at ambient temperature before the image formed by it is normalized. The time span is long due to the large thickness of the meniscus.

Infinity overrun allows full use of the ZM-5A on Nikon cameras with an M42-Nikon lensless adapter.

Unlike its counterparts, the MTO-500, the lens can be used on amateur CZK with a protruding beak of the flash. At least with mine Canon 600D no problems arose.

ZM-5A

ZM-5A

As already noted, the ZM-5A has a replaceable shank, as indicated by the letter “A” in the title. Usually a tail with an M42 thread is mounted on the lens.

It can be noted that the ZM-5A has a successful and reliable construct, but due to poor assembly (there were no Maksutov on them), each lens requires manipulation with loosening the fastening of the overtighted mirror and meniscus.

Optical properties of ZM-5A

ZM-5A has a low aperture with a huge focal length. Because of this, shooting them is a real misery: trembling hands make it difficult to take a sharp shot, and low aperture makes the ISO up. In addition, the lens itself has a low resolution by modern standards. It is simply low due to the becoming noticeable diffraction and residual spherical aberration. Cherry on the cake - low contrast of mirror-lens schemes (a lot of "empty" space - a lot of reflections; it's not for nothing that the coefficient of light scattering is given). Moreover, even on a camera with crop factor 1.6 noticeably serious vignetting.

Moon on the ZM-5A. Crop Post-processing in FS.

Moon on the ZM-5A. Crop Post-processing in FS.

Atmospheric refraction makes a significant contribution to the resulting image - in a bad atmosphere, the quality of photographs of distant objects decreases sharply.

Moon eclipse. Due to the location of the moon directly above the roof of the house, the sharpness is very weak (the effect of atmospheric refraction).

Moon eclipse. Due to the location of the moon directly above the roof of the house, the sharpness is very weak (the effect of atmospheric refraction).

 

"Half" of Venus - 100% crop shot with ZM-5A. So it can be seen in a mediocre amateur telescope, similar to the MTO-1000.

"Half" of Venus - 100% crop shot with ZM-5A. So it can be seen in a mediocre amateur telescope, similar to the MTO-1000.

The ZM-5A has a very, very small depth of field with a small aperture - it is difficult for them to focus through JVI. A lot of photos are getting married.

Due to the presence of central shielding caused by a secondary mirror, the lens has an unusual bokeh - bagels. Not everyone will like it. Subjectively, ZM-5A tries to turn any background into a colorful mess.

Bokeh ZM-5A

Bokeh ZM-5A

The advantage of the lens is a small MDF - only 4 m. For such a FR, this is an achievement. This allows you to shoot a pseudo-macro with this lens, getting frames with a very specific picture. Also, ZM-5A does not suffer from chromatism, unlike lens lenses.

As a result, despite the noted advantages of the lens, the chances of getting a sensible photo on it, say, on a cloudy day, tend to zero. The lens definitely requires a solid tripod for shooting. My attempts to use it for handheld shooting, for photo hunting turned out to be a failure - what the pictures say.

It turned out that the ZM-5A is of much greater interest when used as a telescope. At the same time, small increases of the order of 20-50 times are permissible, which allows us to consider the disks of large (apparently angular size) planets (Jupiter, Saturn, Venus), “split” some binary stars (Lira epsilon is divided into 2 stars [each of them is also double , which can already be seen in more advanced instruments] at 50x - this indicates sufficient optical quality for an astroinstrument), observe large extended nebulae and bright objects of deep space. For use as a telescope, you only need to screw on the M42 macro ring, in which the eyepiece is fixed (for example, from the MBS microscope - affordable and high-quality eyepieces)

Observations in ZM-5A mounted on a heavy tripod Moskinap

Observations in ZM-5A mounted on a heavy tripod Moskinap

For astrophotography, the ZM-5A looks seemingly attractive, but it is not suitable - too small aperture.

Conclusions

Everyone should try the ZM-5A - this is a very inexpensive lens that allows you to feel the super-range. But, unfortunately, it is not worth laying special hopes on him - the image quality is far from what modern cameras require. I would not recommend this lens as the main super telephoto lens. Where the best option for a similar price is the Soviet Tair-3 or even autofocus telezoom. Meanwhile, the ZM-5A is quite suitable for the needs of astronomy - like a small telescope for visual observations.

You will find more reviews from readers of Radozhiva here.

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